Purely electronic mechanism for first-order ferromagnetic transitions: an itinerant electron positive feedback and Fermi surface topological change

摘要

Refrigeration and air conditioning are crucial in modern life and in adapting to climate change. In thiscontext, the behavior of magnetic materials around discontinuous phase transitions has great promisefor new, energy efficient, and environmentally friendly solid-state cooling technologies. Hugeexploitable field-induced entropy and temperature changes typically result from the coupling between amaterial’s spin polarized interacting electrons and the crystal structure, i.e. a magnetostructural effect(Bean and Rodbell, 1962). However, magnetostructurally driven cooling responses are nearly alwaysdegraded by hysteresis. We present a first-principles disordered local moment theory (Gyorffy et al.,1985 and Mendive-Tapia and Staunton, 2019) able to find mechanisms for first-order magnetic phasetransitions which are purely electronic in origin, thus avoiding the need for a magnetostructuralcoupling. We show that this electronic mechanism arises from a mutual feedback between magneticordering, associated to different orientational arrangement of local magnetic moments, and theelectronic structure. In particular, the theory presented is used to explain the hysteresis-free giantcooling properties recently measured in a ferromagnetic divalent rare earth orthorhombic compoundEu2In (Guillou et al., 2019), as shown in panels (a) and (b) in the figure below. It is demonstrated that atopological change of the itinerant electron Fermi surface produces a positive feedback on the magneticinteractions, which in turn generates adiscontinuous character for the paramagnetic-ferromagneticphase transition of Eu2In (see panel (c) of the figure below). Results will be compared with thehexagonal Gd2In counterpart, which exhibits a second order ferromagnetic transition instead and so itwill serve to illustrate and contrast our results. First principles magnetic free energies, heat capacities,Fermi surfaces, density of states, and magnetic local moment interactions, and their crucial dependenceon ferromagnetic order underlying the mechanism, will be shown. This work lays down a groundwork toinvestigate why discontinuous phase transitions of this sort are non-hysteretic as well as to search fornew materials.